Circumpolar Deep Water

The most voluminous water mass in the Southern Ocean is Circumpolar Deep Water (CDW). It extends from -1,400 m to > 3,500 m depth, but it

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Figure 4.5: Hydrographic sections of the salinity (A) and neutral density (B) fields from WOCE Line S2 in the Atlantic sector of the Southern Ocean. Of note is (i) the step-wise ascent of saline, NADW-rich LCDW towards Antarctica where it is capped by fresh, cold AASW in the south and subducting AAIW in the north and (ii) the containment of a large volume of classic Antarctic Bottom Water (AABW; gn>28.27kgm~3) within the Weddell Basin (cf. Fig. 4.4). Hydrographic profiles are derived from the WOCE Southern Ocean Atlas at http://woceatlas.tamu.edu/. Names of fronts are given in Fig. 4.2.

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Weddell Basin

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Weddell Basin

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Figure 4.5: Hydrographic sections of the salinity (A) and neutral density (B) fields from WOCE Line S2 in the Atlantic sector of the Southern Ocean. Of note is (i) the step-wise ascent of saline, NADW-rich LCDW towards Antarctica where it is capped by fresh, cold AASW in the south and subducting AAIW in the north and (ii) the containment of a large volume of classic Antarctic Bottom Water (AABW; gn>28.27kgm~3) within the Weddell Basin (cf. Fig. 4.4). Hydrographic profiles are derived from the WOCE Southern Ocean Atlas at http://woceatlas.tamu.edu/. Names of fronts are given in Fig. 4.2.

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rises to meet AASW or even outcrop along the Antarctic continental margin (Figs. 4.2, 4.4 and 4.5). CDW has two basic types: (1) Upper Circumpolar Deep Water (UCDW) is identified by the oxygen minimum and elevated nutrient concentrations, and has an open ocean depth range of ~ 1,4002,500 m, and (2) Lower Circumpolar Deep Water (LCDW) whose signature is the salinity maximum (34.70-34.75 psu) (Gordon, 1975; Orsi et al., 1995). This maximum reflects the input of NADW that has migrated into the Southern Ocean (Orsi et al., 1995; Rintoul et al., 2001). Upon meeting the ACC, NADW is entrained and transported east around the Antarctic continent, all the while mixing with waters from the Indian and Pacific oceans plus dense waters from Antarctica to form LCDW. Despite the vigorous mixing, the high-salinity signature of NADW is retained (Reid and Lynn, 1971; Gordon, 1975). At several locations around Antarctica, LCDW rises at the continental slope where mixing with super-cold shelf water renews not only the deep circulation of LCDW but also generates Antarctic Bottom Water (AABW), the deepest water mass in the Southern Ocean (Figs. 4.2, 4.4 and 4.5; Foster and Carmack, 1976; Jacobs et al., 1985; Orsi et al., 1999). LCDW is carried equatorwards in all three major oceans by deep western boundary currents (see Section 4.3.3 and Schmitz, 1995; Hogg, 2001). According to Rintoul et al. (2001) mixing with fresher waters, together with the biological depletion of oxygen, slowly modify LCDW into a less dense, lower oxygenated variant that returns south as UCDW. In some WOCE sections, such as P15 in the Indian Ocean, oxygen depletion of UCDW may be influenced by the direct injection of nutrient-enriched deep waters from the North Indian and North Pacific oceans.

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